[BlueMondaySkipper]

Definition of delta (Entry 1 of 5)
1 : the 4th letter of the Greek alphabet
— see ALPHABET TABLE
2 : something shaped like a capital Greek delta
especially, geology : the alluvial deposit at the mouth of a river
the Mississippi Delta
3 mathematics : an increment of a variable —symbol Δ

So I’m the case of Q, Delta would mean an increment

[xone]

Google ‘Delta definition difference’, that’s the context.